There are a variety of different techniques for bleaching of say a kraft pulp that employ either sodium or calcium hypochlorite. Both processes have their respective advantages. The use of NaOCl permits easier process control, minimizes scaling, plugging and process upsets, and lowers maintenance costs.
The use of calcium based hypochlorite (Ca(OCl.sub.2) has the advantage of providing a significant improvement in drainage rate of the pulp thereby increasing production significantly.
When operating with sodium hypochlorite as a bleaching agent, attempts have been made to increase the drainage rate and thereby the production of the machine by adding drainage aids. Alum has proven effective to improve drainage but is undesirable as it depresses the pH level to the point where corrosion in the machine tends to occur and many customers have found the presence of alum in the product to be detrimental.
Other drainage aids have been tried and found ineffective or too costly to provide a commercial advantage.
It is also known to add Ca.sup.++ ions to the head box to improve drainage, however the amount of Ca.sup.++ ions necessary to show some improvement is significant and the Cl.sup.- ions that are carried to the forming machine result in a significant corrosion problem.
In all of the above cases where drainage improvement has been sought the additive is applied in the head box so that the Na.sup.+ and Cl.sup.- ions are also present when the pulp as it is being formed.